IR absorbing reflector

ABSTRACT

A infrared (IR) light absorbing reflector is formed with a substrate that supports a first IR absorptive multilayer part having multiple layers of partial IR absorbing thin films. The first IR absorptive multi-layer part supports a second visible light reflecting multilayer part.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 11/179,117, filed Jul. 12,2005, now U.S. Pat. No. 7,349,151.

BACKGROUND

Optical systems for medical, theatrical, educational and other purposesinvolve the projection of high intensity light beams. One problem oftenarises in eliminating harmful effects of infra-red radiation. Use hasbeen made of mirrors, known as cold-light mirrors, whose reflectance isrestricted to limited wavelength bands. These produce the requiredintensity of light in the reflected beam.

Many different types of cold-light mirrors have been produced. Someutilize all polymer construction. Others form multiple dielectric layerson a glass substrate. One such device involves the use of a pigmentedvitreous layer formed on a metal substrate in combination with multiplelayers of quarter wavelength having alternating high and low indices. Adecoupling layer may also be required between the pigmented layer andthe multiple layers. Prior cold-light mirrors suffer from poor heatmanagement capabilities, or difficulty in manufacture due to theinability to form different types of layers in a single processingmachine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a cold-light reflector constructedaccording to an example embodiment.

FIG. 2 is a table identifying layers of the cold-light reflector of FIG.1 according to an example embodiment.

FIG. 3 is a table identifying alternative layers of the cold-lightreflector of FIG. 1 according to an example embodiment.

FIG. 4 is a table identifying further alternative layers of thecold-light reflector of FIG. 1 according to an example embodiment.

FIG. 5 is a block schematic diagram of a cold-light reflector of FIG. 1used to project beams of bandpass filtered light according to an exampleembodiment.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that structural, logical andelectrical changes may be made without departing from the scope of thepresent invention. The following description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined by the appended claims.

FIG. 1 illustrates a cold-light IR absorbing reflector generally at 100.The reflector is formed on a substrate 110 in one embodiment. Thereflector may be designed to bandpass visible light or to reflect non-IRlight. Substrate 110 may be formed of any material compatible withfabrication methods used to form IR absorbing reflector 100. In oneembodiment, substrate 110 is metallic, and may be formed of Al, or mostother materials compatible with fabrication methods used to form the IRabsorbing reflector 100. A Multi-layer QWT (Quarter WavelengthThickness) is a general term for optical thin film stack. The actualthickness of multi-layer does not have to be exactly ¼ of wavelength. Ingeneral, the thickness of each layer may vary from ¼ wavelength to 1wavelength. A Multi-layer QWT stack is formed on the substrate in twoparts, bottom part 120 and upper part 130. A first IR (infrared)absorptive multi-layer part 120 is formed and supported by thesubstrate. In one embodiment, part 120 comprises a bottom part of themulti-layer QWT stack and has IR absorptive index and thickness matchedlayers. The IR absorptive matching layers comprise multiple layers ofdielectric, metal, or semi-metal thin film materials, where in generalthe differences in indices of refraction (hereafter “indices” forbrevity) of the layers are selected to be small to minimize reflections.The IR absorptive matching layers in one embodiment use IR materials(i.e., W, Ni, Ti, Ta, Si, Al₂O₃, Cr₂O₃, and SiO₂). The IR absorptivematching layers provide low reflectance for most of the IR region (1μm˜20 μm).

The reflectance of IR in this layer may be reduced by selectingdifferential indices of adjacent layers to be small. This is referred toas index matching. Further reduction of the reflectance of this layercan be obtained by alternating the thickness of each layer. In oneembodiment, layer thicknesses may be determined by computer simulation,optimizing each layer to obtain desired properties, such as absorptionof particular wavelengths of IR to obtain an overall absorption that issubstantially uniform from the near IR to the far IR.

The term QWT encompasses thicknesses that are generally one quarter ofthe wavelength of visible light. Thicknesses may be varied to providedifferent desired properties, and generally range down to one third ofthe wavelength of visible light. Common thicknesses of the layers areshown in the examples of FIGS. 2, 3 and 4. Other thicknesses may also beused to obtain desired characteristics of reflection of visible lightand absorption of radiation outside the desired visible light range.

The upper multi-layer QWT part 130 may include some dielectric thin filmmaterials (i.e., TiO₂, SiO₂, Ta₂O₃, Al₂O₃, Nb₂O₅, HfO_(x) and ZrO₂). Thedielectric layers include alternate relative high and low indexmaterial. Part 130 has alternating layers with large differentialindices. In one embodiment, the high index material is as high aspossible and the low index material is as low as possible. The contrastin indices between alternating layers may maximize reflections in thevisible band of light, while minimizing reflections in non-visible bandsof light. Part 130 of the QWT layers provides a high reflectance(greater than 95%) for visible radiation (400 nm˜800 nm) and absorbs UV(ultraviolet) radiation. Thicknesses may be determined via an iterativecomputer simulation that is provided a target reflectivity. TiO₂, as alayer in either of the parts provides absorption of UV.

In one embodiment, the layers of the stack comprise multi-layers ofdielectric, semi-metal, and metal quarter wavelength thin film to directcoat on the top of metal substrate. An absorption layer, such as apigmented vitreous layer is not needed. This allows the optical thinfilm layers to be formed using a single coating process, such asphysical vapor deposition (PVD) or chemical vapor deposition (CVD) in asingle coating machine, which may reduce the complexity ofmanufacturing. The use of semi-metal and metal thin films in part 120 ofthe stack can provide coefficient of thermal expansion (CTE) matchingbetween the dielectric bandpass reflective layers, part 130, and themetal substrate 110. The IR absorptive matching layers 120 may in effectact as a thermal expansion absorber.

In a further embodiment, a suitable thin adhesion layer 112 may beformed between part 120 and part 110. Such an adhesion layer 112 mayalso perform some amount of IR and/or UV non-visible light or incidentvisible light absorption and also provide for better adhesion of thedielectric layers to a metal surface.

FIG. 2 is a table identifying layers of the cold-light reflector of FIG.1 according to an example embodiment. In this embodiment, the firsttwenty-five layers, corresponding to the bandpass reflective layers inpart 130, are alternating layers of TiO₂ and SiO₂. In variousembodiments, approximately 20 to 40 of such layers may be used. The TiO₂layers have a refractive index of 2.5 in one embodiment, and a physicalthickness of between approximately 19 nm and approximately 63 nm. TheTiO₂ layers also have an extinction coefficient of approximately0.00004, which refers to the fraction of light lost to scattering andabsorption per unit distance, expressed as a fraction per meter. Theexample embodiments shown in the Figures may specify refractive indices,thicknesses and extinction coefficients to a fairly high degree ofresolution. It should be understood that these represent preciseembodiment examples, and that the values may vary significantly from theexamples in further embodiments, while still providing desiredproperties of reflecting varying amounts of visible light and absorbingdifferent amounts of UV and IR light.

The SiO₂ layers have refractive index of approximately 1.47 at visibleregion, and vary between approximately 79 and 134 nm in thickness, Thenumber of alternating layers in part 130 may be varied in differentembodiments to provide different reflective characteristics.

Eight layers of alternating Ti and SiO₂ are used to form the IRabsorbing layers of part 120. The refractive indices are 1.7 and 1.5respectively at a wavelength of approximately 510 nm corresponding to anapproximate middle of the visible spectrum. This provides a sufficientmatch to minimize reflectance of the IR. The thicknesses range fromapproximately 9 to 68 nm for the Ti layers and approximately 193 to 357nm for the SiO₂ layers. The Ti layers have an extinction coefficient ofapproximately 2.3. The IR absorbing layers are formed directly on an Alsubstrate. The dimensions of the substrate are much thicker than thethin film layers. The substrate may be formed of other materials ifdesired, and the layers may be varied in material to provide suitablethermal expansion characteristics, as well as IR absorbingcharacteristics. In one embodiment, the substrate provides a heat sinkfunction to handle heat generated from the IR.

FIG. 3 is a table identifying alternative layers of the cold-lightreflector of FIG. 1 according to an example embodiment. Part 130 in thisembodiment is formed of twenty-five alternating layers of TiO₂ and SiO₂.The refractive index of the TiO₂ is approximate 2.5, and that of theSiO₂ is approximately 1.5. The TiO₂ also has an extinction coefficientof 0.00002. Thicknesses may be varied significantly from those in theprevious example of FIG. 2. Part 120 in this embodiment is formed ofnine layers of Ni (refractive index of approximately 1.7) and SiO₂(refractive index of approximately 1.5. The thicknesses of the Ni layersvary from approximately 3 to 30 nm, and the thickness of the SiO₂ layersvary from approximately 716 to 452 nm. The Ni layers also have anextinction coefficient of approximately 3.02. An aluminum substrate isagain used, with an extinction coefficient of approximately 6.2 andrefractive index of approximately 0.8.

FIG. 4 is a table identifying further alternative layers of thecold-light reflector of FIG. 1 according to an example embodiment. Inthis embodiment, alternating layers of TiO₂ and SiO₂ are again used forpart 130. The TiO₂ layers vary in thickness between approximately 45 and383 nm at the interface to the layers of part 120. The TiO₂ layers havea refractive index of approximately 2.5 and an extinction coefficient ofapproximately 0.00002. The SiO₂ layers vary in thickness betweenapproximately 78 to 132 nm and have a refractive index of approximately1.5 and an extinction coefficient of approximately 0.

Part 120 in this embodiment is comprised often alternating layers of Wand Cr₂O₃. The W layers vary in thickness from approximately 10 to 18 nmand have an index of approximately 3.4 and extinction coefficient ofapproximately 2.7. The Cr₂O₃ layers are approximately 382 nm thick witha refractive index of approximately 2.2 and an extinction coefficient ofapproximately 0.07. The layers of part 120 are formed directly on analuminum substrate having a refractive index of approximately 0.8 and anextinction coefficient of approximately 6.2.

FIG. 5 is a block schematic diagram of a light beam projector 500 usedto project beams of bandpass filtered light according to an exampleembodiment. The cold-light IR absorbing reflector in this embodiment isconcaved-shaped such as in a parabola or in an elliptical form toprovide for reflection of visible light from a light source 505 toward alens 520, which may be supported by a frame 510 coupling the reflectorto the lens. Lens 520 can also to be a glass or semi-metal window tohold the gas of a light source. Lens 520 may be designed to focus thelight in a desired direction, or may simply pass the light through. Inone embodiment, the reflector is coupled directly to the lens 520without the need for frame 510. In a further embodiment, heat resultingfrom absorption of IR may be dissipated by an integral heat removallayer or device 530, thermally coupled to the IR absorption layer, part120, such as through substrate 110. In various embodiments, device 530may be a heat sink or heat pipe system. In further embodiments, device530 may be the same layer as the substrate, formed thick enough toaccomplish desired heat transfer characteristics.

1. An infrared (IR) absorbing reflector comprising: a substrate; a firstmultilayer Quarter Wavelength Thickness (QWT) part having multiple setsof alternating layers of partial infrared (IR) absorbing thin filmssupported by the substrate; and a second multilayer QWT part havingmultiple sets of alternating layers that reflect visible light supportedby the first multilayer part, wherein the second multilayer QWT partcomprises dielectric layers of alternate relative high and low indexmaterial, and the dielectric layers comprise approximately 20 to 40layers.
 2. The IR absorbing reflector of claim 1 wherein the dielectriclayers absorb UV (ultraviolet radiation).
 3. The IR absorbing reflectorof claim 2 wherein the dielectric layers provide a reflectance of atleast approximately 95% in the visible spectrum (400 nm˜800 nm).
 4. TheIR absorbing reflector of claim 1 wherein the dielectric layers areselected from the group consisting of TiO₂, SiO₂, ZrO₂, HfO_(x), Ta₂O₃,Nb₂O₅, and Al₂O₃.
 5. The IR absorbing reflector of claim 1 wherein thefirst multilayer QWT part comprises nine layers of alternating Ni andSiO₂.
 6. The IR absorbing reflector of claim 1 wherein the firstmultilayer QWT part comprises ten layers of alternating W and Cr₂O₃. 7.The IR absorbing reflector of claim 1 wherein the first multilayer QWTpart comprises at least eight layers of alternating dielectric, metal,or semi-metal thin film materials.
 8. An infrared (IR) absorbingreflector comprising: a substrate; a first multilayer Quarter WavelengthThickness (QWT) part having multiple sets of alternating layers ofpartial infrared (IR) absorbing thin films supported by the substrate;and a second multilayer QWT part having multiple sets of alternatinglayers that reflect visible light supported by the first multilayerpart, wherein the first multilayer QWT part comprises eight layers ofalternating Ti and SiO₂.
 9. An infrared (IR) absorbing reflectorcomprising: a substrate; means for absorbing particular wavelengths ofIR (infrared) to obtain an overall absorption that is substantiallyuniform from a near IR to a far IR disposed on the substrate andincluding multiple sets of two alternating layers of IR absorbing films;and means for reflecting visible light while passing IR to the means forabsorbing and including multiple alternating layers of TiO₂ and SiO₂.10. The IR absorbing reflector of claim 9 wherein the means forabsorbing includes eight layers of alternating dielectric, metal, orsemi-metal thin film materials.
 11. The IR absorbing reflector of claim9 wherein the means for absorbing includes eight layers of alternatingTi and SiO₂.
 12. An infrared (IR) absorbing reflector comprising: asubstrate; an IR (infrared) absorptive multilayer part includingmultiple sets of alternating layers of deposited thin-films formeddirectly on the substrate; and a visible light reflecting, IRtransmissive, and UV (ultraviolet) absorbing multilayer part formeddirectly on the IR absorptive multilayer part, wherein the IR absorptivemultilayer part includes eight layers of alternating dielectric, metal,or semi-metal thin film materials.
 13. A method comprising: forming anIR (infrared) absorptive part of multiple sets of alternating layers ofthin-films supported by a substrate; and forming a visible lightreflecting part of multiple sets of alternating layers of thin-filmssupported by the IR absorptive part, wherein the IR absorptive partcomprises eight layers of alternating Ti and SiO₂.
 14. The method ofclaim 13 wherein the visible light reflecting part comprises dielectriclayers of alternate relative high and low index material.
 15. The methodof claim 14 wherein the dielectric layers include UV (ultraviolet)absorptive material.
 16. The method of claim 14 wherein the dielectriclayers provide a reflectance of at least approximately 95% in thevisible region.
 17. The method of claim 14 wherein the dielectric layersare selected from the group consisting of TiO₂, SiO₂, Ta₂O₃, HfO_(x),Al₂O₃, Nb₂O₅, and ZrO₂.